A power supply monitoring IC monitors the voltage of a cell such as a lithium-ion cell and performs control operations necessary to prevent the cell from being brought into an overcharged or overdischarged state. For example, when the voltage of the cell becomes higher than an overcharge voltage, the power supply monitoring IC outputs a control signal to turn off a switching device connected in series with the cell, and thereby inhibits the charging of the cell. With a lithium-ion cell, the overcharge voltage is, for example, 4.2 V.
However, the voltage (i.e. the detected voltage) of the cell, when the cell is charged to close to the overcharge voltage, may temporarily exceed the overcharge voltage because of noise or the like. In such a case, if the power supply monitoring IC in response thereto immediately outputs the control signal, erroneous detection results. This makes it impossible to charge the cell so fully as to have a voltage close enough to the overcharge voltage, and thus causes the cell to last for an accordingly shorter length of time than it should. For this reason, as shown in FIG. 3, the power supply monitoring IC is provided with a non-responsive interval setting circuit for setting a non-responsive interval in which it does not respond to noise (i.e. the interval in which it does not output the control signal in response to noise). Thus, only when the detected voltage remains higher than the overcharge voltage for a longer period of time than the non-responsive interval, does the power supply monitoring IC output the control signal. In this way, erroneous detection is prevented.
In FIG. 3, a constant current source 1, which outputs a current 11, is connected through a switching device 2 to the base of a transistor 3c. When the voltage of the cell exceeds the overcharge voltage, a high-level signal S1 is applied to the switching device. When this signal S1 is applied to the switching device 2, the switching device 2 is turned off. As the switching device 2, a switching transistor or the like is used. The transistor 3c has its emitter connected to ground, and has its collector connected to the constant current source 4.
Between the collector of the transistor 3c and ground, a capacitor 5 for setting the non-responsive interval is connected. To detect the voltage across the capacitor 5, the non-inverting input terminal (+) of a comparator 6 is connected to the collector of the transistor 3c, and to the inverting input terminal (-) of the comparator 6 is fed a voltage higher than the ground level by a non-responsive interval setting voltage Vref.
According to this circuit configuration, when the voltage of the cell becomes higher than the overdischarge voltage, the signal S1 is fed. Then, the switching device 2 is turned off, and thus the transistor 3c is turned off. Then, the current I2 from the constant current source 4 is fed to the capacitor 5 to charge it. This causes the voltage Vc across the capacitor 5 to increase linearly. The comparator 6 compares this voltage Vc with a predetermined non-responsive interval setting voltage Vref, and outputs a high level if the voltage Vc is higher than Vref or a low level if the voltage Vc is lower than Vref.
Now, consider a case where noise happens to generate the signal S1. Then, the switching device 2 is turned off, and thus the transistor 3c is turned off. As a result, the capacitor 5 starts being charged. However, since noise usually lasts for a short period of time (i.e. is narrow in width), the signal S1 disappears in a moment, turning the switching device 2 and the transistor 3c on. Thus, the voltage Vc across the capacitor 5 does not become higher than the voltage Vref. This case is illustrated in FIG. 4. In FIG. 4, VH represents the overcharge voltage, Vi represents the input voltage (in this case, the composite voltage of the cell voltage and the noise). When the transistor 3c is on, its collector current .beta..multidot.I1 (where .beta. represents the current amplification factor of the transistor 3c) is allowed to flow, and thus the capacitor 5 is discharged. In this way, even if the detected voltage temporarily exceeds the overcharge voltage because of noise or the like, the comparator 6 does not output a signal S2.
By contrast, as illustrated in FIG. 5, when the detected voltage remains higher than the overcharge voltage for a longer period of time than an interval T1, the signal S1 keeps the switching device 2 off and the transistor 3c on for a sufficiently long period of time to allow the voltage across the capacitor 5 to rise. When the voltage Vc becomes higher than the voltage Vref, the power supply monitoring IC outputs the signal S2 as a control signal. This interval T1 is the on-responsive interval, which is given by the following formula: EQU T1=C.multidot.(Vref-Vsat)/I2 EQU .apprxeq.C.multidot.Vref/I2
(where C represents the capacitance of the capacitor 5, Vsat represents the collector-emitter voltage of the transistor 3c when the capacitor 5 is discharged, and Vsat.apprxeq.0)
However, in this conventional power supply monitoring IC, as shown in FIG. 6, which is an enlarged version of the waveform diagram shown in FIG. 4, the discharging of the capacitor requires an interval T2, and this interval T2 is difficult to reduce to as short as a several-hundredth to a several-thousandth of the charging interval T1. For this reason, in an appliance such as a portable telephone that incorporates a high-frequency clock, the input voltage Vi may exhibit a waveform as shown in FIG. 7 because of continuous high-frequency noise. In such a case, the capacitor 5 starts being charged before it is discharged completely, and therefore the voltage Vc gradually rises until, at a point of time t1, it reaches the voltage Vref. As a result, the comparator 6 outputs a high level as the signal S2. In this way, high-frequency noise tends to cause malfunctioning.
To prevent such malfunctioning, it is necessary, as shown in FIG. 8, to maximize the discharging rate of the capacitor 5 and thereby minimize the period T2 (see FIG. 6). However, in the conventional power supply monitoring IC described above, the discharging is achieved by the collector current .beta..multidot.I1 of the transistor 3c, and therefore, to increase the discharging rate, it is inevitable to increase the current I1. This inconveniently increases the current consumption of the power supply monitoring IC. Since the power supply monitoring IC performs its monitoring operations by using the current fed from the cell, an increase in its current consumption is undesirable.
Moreover, the voltage Vsat depends on temperature, and thus manufacturing- and temperature-related variations in the characteristics of circuit elements used lead to inaccurate setting of the non-responsive interval.